1. Field of the Invention
This invention relates to an electrohydraulic device for controlling the engagement of the clutch in motor vehicles and the like.
In motor vehicles, the clutch is generally controlled by a pedal which by overcoming the reaction of one or more springs causes the clutch plate, rigid with the main gearbox shaft, to separate from the flywheel, which is rigid with the crankshaft (clutch disengaged). On releasing the pedal the elastic reaction of the previously loaded springs prevails to again bring the plate into contact with the flywheel, so restoring their torsional connection (clutch engaged).
To provide a servo-control for both clutch release and engagement, and thus make clutch control easier and quicker, and particularly suitable for invalids without the use of a lower limb, it has been proposed to utilize the vacuum of the engine when in operation.
Specifically, the clutch lever is provided with a switch for controlling one or more solenoid valves, which use the engine vacuum to control the clutch operation. In practice the driver has merely to operate the gear lever in order to automatically disengage and then re-engage the clutch thereby.
Although this known method solves the general clutch servo-control problem, it has certain limits and drawbacks, and in particular:
its action is not totally reliable under all conditions. In particular, when the engine is idling (for example in queues) it is unable to create the necessary vacuum for repeated clutch operation;
if the engine should accidentally stall (for example when maneuvering or because of low temperature) there may not be sufficient vacuum to release the clutch and thus disengage the gear;
there is a certain delay in its action, which in some situations can make the vehicle difficult to control. This is particularly the case in the so-called "pavement-window test", in which the front of a vehicle faces a pavement step, beyond which there is an obstacle such as a window. the need to bring the engine up to high speed to enable the vehicle to climb the step means that the clutch cannot be immediately released after this climb in order to halt the vehicle before collision with the obstacle. A further example in which the negative consequences of a delay in the release of the clutch are evident is in automobile competitions, in which this delay obliges "short" entry into a bend (i.e. with anticipated gear change), which can result in precious fractions of a second lost;
unsmooth and insensitive control, in that the clutch engagement is practically uncontrollable by the driver, being totally dependent on vacuum control, and smooth, slow movements such as those generally required when parking or when close to obstacles are not possible.
2. Discussion of the Prior Art
Control systems of hydraulic or electromagnetic type have also been studied. These have however been used up to the present time only in experimental form on racing vehicles and with considerable operational limitations.
DE-A-3 306 519 discloses a hydraulic actuator for controlling clutch engagement and release, a hydraulic circuit for controlling the actuator via an interceptor, an electronic control unit to control the interceptor to feed the hydraulic actuator to thus cause clutch release, and to enable the actuator to discharge, one or more solenoid valves connected into the discharge circuit of the hydraulic actuator and caused to open by the control unit by a sequence of pulses related to the rotational speed of the vehicle engine. The command to the interceptor for feeding the hydraulic actuator is provided by the control unit automatically when predetermined engine operating conditions exist, and/or by the action of the driver.
EP-A-0 254 483 discloses a fluid pressure regulator valve comprising a linear solenoid that acts on an asymmetric spool valve to selectively communicate one port with another to thereby regulate fluid pressure.